V-8 GM Engine - Your Questions Answered

During the course of the past few months, some of you have sent letters to Circle Track either posing questions or raising issues worthy of further discussion. This month, we elected to share a couple of them, along with various comments directed to clarification or stimulate additional thought.

In reference to the May '09 Enginology column, a reader wanted further discussion about "tuning" engines on an individual cylinder-to-cylinder basis; e.g., focusing on treating cylinders or groups of cylinders as "engines" unto themselves. You may recall we suggested, aside from the "cross-talk" that goes on among cylinders linked by way of a collected exhaust system or common-plane intake manifold, it's possible to adjust intake/exhaust paths and valve timing to "tailor" torque curves to specific track conditions. (You can even provide individual cylinder ignition spark timing, but that's a subject for possible later review.) Well, maybe that's too general but it describes the concept. The reader appeared to want more information about how this might apply to race classes requiring use of stock intake manifolds, referencing a recall he had about using 1.6 rockers on an engine's No. 1, 2, 7, and 9 cylinders in combination with 1.5 rockers on cylinders No. 4, 6, 3, and 5. I'm going to assume he was dealing with a V-8 GM engine and a firing order of 1-8-4-3-7-5-7-2.

For purposes of discussion, visualize a single-plane intake manifold on a V-8-type engine of this firing sequence. You could apply what follows to a two-plane design, but results would be less significant. Now, all else being equal, shorter intake (or exhaust) passages tend to "tune" to higher rpm than longer ones. Based on our example manifold, the inboard four runners (cylinders Nos. 4, 6, 3, and 5) would tune to higher rpm than the outboard four runners (Nos. 1, 7, 2, and 8). Of the options this poses, one is to treat these two "sets" of cylinders in a different tuning fashion, with respect to lengths of intake/exhaust passages and valve events (lift, duration, lobe separation, and phasing). As you'd expect, rocker arm ratio plays into this.

If I recall correctly, applying rocker ratios to the specific cylinders the reader indicated tends to affect where power is produced but not in a fashion that addresses intake-passage length. His pattern is the reverse of what you'd do to optimize runner length. Instead, the approach he mentioned tends to compromise the difference in intake runner lengths by increasing the net time each longer runner contributes to the inlet cycle and less time for the shorter ones, likely with the intent of narrowing the effective rpm range of most efficient operation. It would be akin to making the longer passages work better at higher rpm and the shorter ones at lower engine speed. This compromise, and that's what it amounts to, would be intended for engines operated in a comparatively narrow span of rpm; e.g. encourage the longer runners to perform better at high rpm and the shorter ones at lower rpm. Actually, both sets would thus be discouraged to work best where their geometry suggests.

On the other hand, if we'd like to take advantage of intake manifold passages already determined by a part's design, reversing the approach to optimize the length benefits of the inboard and outboard passages has proven more beneficial, especially in a broader range of rpm, like off-the-corner torque and increased speed at the flag stand. Unfortunately, when designing the majority of racing engine components, compromise is unavoidable, whether it relates to structural, material, manufacturing, or functional requirements. Once designed, however, there are "adjustments" to functionality that can be made, and tailoring valve events to intake and exhaust flow characteristics is one area for exploration.

Another reader recalled a story Smokey provided CT back in 1984, dealing with his "hot vapor" engine. Although signed "anonymous," the writer posed the question about "why all these great ideas to improve engine performance rarely make it into production." Rather than take issue with this statement, we prefer to reference some material previously provided in this little monthly column. But first a couple of thoughts.

There isn't necessarily a disconnect between the OEM and landscape of "inventors" that populate the automotive community. In fact, including those provided by Smokey, there are numerous concepts either pioneered or largely developed in the motorsports arena that found a home in OEM-produced vehicles. That's a matter of record. While his "hot vapor" engine produced an array of combustion efficiency benefits, it would've been (or was believed to have been by the OEM who reviewed his findings at the time) a significant economic step and commitment from then-conventional powerplant technologies. There's no doubt the concept worked. I was present during some of the dyno tests and drove the car. In fact, I recently learned it's still around and functional.

The point is that as the convergence of fuel prices, environmental regulations, on-board electronics, and a host of related technologies evolved into their present and collective state, the essence of efficient combustion is undergoing significant study at levels ranging from the academia to the OEM. Fundamental postulations advanced from Sir Harry Ricardo (in the 1940s) to today deal with air/fuel charge conditioning, both before and during combustion, are still the basis for how we currently view this process.

Over time and on the pages of this magazine, we've touched upon the need for and simplified ways racing engine builders can both understand and address optimizing combustion efficiency in their engines . . . and we plan to continue doing that. The fact of the matter is that once combustion is initiated and moves past the ignition delay period, a so-called "chain reaction" continues for the duration of a given combustion cycle. If liquid fuel droplets aren't both reduced in size and caused to be more uniform in size, both the rate of this reaction we tend to loosely call "burning," and its completeness can be penalized. Unwanted pressure spikes and otherwise "uncontrolled combustion" can lead to numerous problems, including damaged parts and lost power. So, by whatever means at our disposal to break down fuel droplet size, it's critically important to do so. Since we're dealing with very small time increments to complete a given combustion cycle (made less so as engine speed increases), the goal of finely-atomized fuel is acute to optimizing brake power.

As racers, we tend to look for quick and easy ways to address the fuel atomization issue, knowing that the smaller the droplets, the better the chance of achieving efficient homogenization of air/fuel charges and improved combustion. While the heated air in Smokey's "hot vapor" experiment introduced air with less oxygen content (by volume), this was amply offset by significant improvements in fuel atomization that resulted in reduced emissions and measureable power increases, each of which occurred at lower rpm.

For street-driven engines that address both emissions and fuel economy (fewer grams/mile based on a reduction in net engine airflow), this was a plus. In a racing engine where we'd like to increase the oxygen content of inlet air (cooler is better) in combination with improved fuel atomization, we need to explore other means. Especially for engines not fitted with fuel injectors that inherently improve atomization beyond that of carburetors, seeking ways to mechanically break down liquid fuel is an option . . . which brings us right back to our previous discussion about wet-flow surfaces, increased carburetor efficiency, proper airflow quality, and improved means to reduce or eliminate air/fuel separation, both in the inlet track and combustion space.

Ideally, we'd like to combine the benefits of good air/fuel homogeneity and "generated" turbulence. In particular, the latter condition tends to accelerate the combustion process which, if accomplished in a controlled fashion, can enable a reduction in initial spark ignition timing and a corresponding increase in net torque at the flywheel. This concept has also been discussed in one of our previous CT offerings.

In addition, there have been fuel conditioning systems that employ ultra-sonic wave activity; an example provided by the use of piezoelectric discs through which pre-combustion liquid fuel was passed to mechanically produce ultra-small droplets. Such technology has not proven successful for racing applications but could possibly be adapted through further research and refinement. The results I've seen from such experiments were dramatic to the extent power was sustainable from air/fuel mixture ratios much leaner than typical of conventional atomization methods (e.g., fuel injection, carburetion, and so on). All this is simply validation of the fact improved fuel atomization can lead to faster combustion, reduced spark timing, and increased power. Obviously, from the practical perspective of a racer, that's a goal.